Electrophotographic process

ABSTRACT

An electrophotographic process comprising forming an electrostatic latent image on a photoconductive insulating coating and developing said latent image by liquid development where the photoconductive coating and a development electrode are brought into direct contact with each other during development. The coating and/or the development electrode have uniform surface irregularities of very fine structure such that the directly contacted area of the coating with the electrode does not exceed 10 percent of the whole area of the processed surface to be developed where the majority of the contact points having a representative size not greater than 50 microns. The two surfaces undergo no movement during the contact.

United States Patent Sato et al.

151 3,685,907 1451 Aug. 22, 1972 [54] ELECTROPHOTOGRAPHIC PROCESS [72] Inventors: Masamichi Sato; Osamu Fukushima; Haiime Miyazuka; Masaaki Takimoto; Seiii Matsumoto; Yasuo Tamai; Satoru l-londo, all of 105, Ohaza, Mizonuma, Asaka-shi, Saitama, Japan [22] Filed: June 25, 1970 [21] Appl. N0.: 49,793

[30] Foreign Application Priority Data June 25', 1969 Japan ..44/501 16 52 us. c1. ..355/17, 117/934, 117/9342, 1 18/625, 355/10 511 1m. (:1. ..G02g 13/00 53 Field of Search ..355 10, 17; 117/934, 9342; 118/625 [56] References Cited UNITED STATES PATENTS 3,034,043 4/1963 Gundlach ..355/10 x FOREIGN PATENTS OR APPLICATIONS 801,687 l2/l968 Canada ..355/l0 Primary Examiner-Samuel S. Matthews Assistant Examiner-Richard A. Wintercorn Att0rney-Addams and Ferguson [57] ABSTRACT An electrophotographic process comprising forming an electrostatic latent image on a photoconductive insulating coating and developing said latent image by liquid development where the photoconductive coating and a development electrode are brought into direct contact with each other during development. The coating and/or the development electrode have uniform surface irregularities of very fine structure such that the directly contacted area of the coating with the electrode does not exceed 10 percent of the whole area of the processed surface to be developed where the majority of the contact points having a representative size not greater than 50 microns. The two surfaces undergo no movement during the con tact.

10 Claims, 7 Drawing Figures ELECTROPHOTOGRAPHIC PROCESS This invention relates to an improved process of electrophotography, especially suitable for reproduction of continuous tone images.

Xerography which comprises forming an electrostatic latent image on a photoconductive insulating layer, converting said image into a visible one with the use of a fine toner having an electrostatic charge of a suitable polarity relative to said latent image has accepted a great success in the field of document copying wherein line images are to be reproduced. Of course, xerography is capable of reproducing continuous toner images satisfactorily; for that purpose, development is carried out employing an electrical conductor referred to as development electrode which is placed close to the electrophotographic layer to be developed so as to attract electrical lines of force to the conductor thus eliminating so-called edge effect. It is also known that the finer becomes the toner used, the more faithfully is a continuous tone image reproduced. Hence the best result is obtained by electrophoretic or liquid development wherein the finest toner at present is available.

Most of the conventional liquid development failed to produce an image having a toner density distribution proportional to that of the charge density in a latent electrostatic image, mainly because they could not utilize a development electrode which was placed an infinitely small distance apart from the surface to be developed. Thus, the so-called edge effect was clearly observed in the resulting image at those areas where the charge density sharply changed laterally. In addition to such defect, streaks sometimes appeared in the direction of relative movement between the developed surface and the developer liquid. Accordingly continuous tone images produced by liquid development were far inferior in quality to those of silver halide printing paper or film which are now prevailing commercially.

We have succeeded in noticeably improving the image quality of electrophotographic prints by bringing a development electrode into direct contact with the photoconductive surface to be developed. Further an extensive study on a wide variety of liquid developer, photoconductive recording material and development electrode for such development procedure led to an conclusion that the most desirable prints can be obtained when the electrode and the surface bearing an electrostatic latent image are contacted with each other at many points of very small area scattered throughout the surface.

Accordingly, this invention relates to an electrophotographic process utilizing liquid development suitable for accurate reproduction of continuous tone image.

This invention provides an electrophotographic process comprising developing an electrostatic latent image formed on an electrophotographic photoconductive coating by liquid development whereby the coating to be developed and a development electrode is in direct contact with each other, characterized by that the coating and/or the development electrode have uniform surface irregularities of very fine structure in such a manner that the directly contacted area with said electrode does not exceed percent in the whole area of the surface to be developed and that the two surfaces undergo no relative movement during such contact.

. The present invention will now be described in more detail referring to the accompanying drawings in which FIG. 1 to FIG. 5 illustrate schematic cross-sectional views of different types of processing apparatus to carry out the present process, and FIG. 6 illustrates schematic cross-sectional view of conventional processing apparatus. FIG. 7 illustrates a photoconductive layer having raised contact points in accordance with the invention.

In FIG. 1, four rotatable rollers 101-104 work as development electrode; each of them is placed in contact with another roller designated as 111-114, the latter rollers will be referred to as carrier rollers. Cleaning pads 121-124 are provided on the development electrode rollers in pressure contact therewith. A liquid developer supply nozzle (131-134) is set up above each of the cleaning pad, blowing out the developer onto the pad.

The development electrode rollers are preferably made of metal including stainless steel, steel, brass, duralrnine, etc. A very thin film of insulating or semiconductive oxide formed by anodizing or acid treatment may be provided on them. Rollers of a wide range of diameter are available, which may be 1 mm to several 10 cm in general, but more preferably may be from several mm to several cm. The number of the development electrode rollers used may be determined by taking the development velocity into account; for a lower velocity, i.e., circular velocity of the roller, a small number of rollers are needed, and for a higher velocity the number must be increased. Practically several ten rolls are used. Carrier rollers may also be metallic, but more preferably such as those covered with rubber or sponge.

Cleaning pads are made of soft, and fibrous or porous material such as sponge, felt, flannel, or paper.

The development electrode rollers are driven by a motor through gear, chain, belt, or friction roller; the carrier rollers, may be driven frictionally by the development rollers, or by gear, chain, belt, etc. Alternatively, the carrier rollers may be driven by a motor, and the development electrode rollers may be driven by the carrier rollers. The development electrode rollers and the carrier rollers may be pressed mutually, or the former may be placed on the latter by its own weight and rotated frictionally.

An electrophotographic recording sheet is fed into this apparatus with its recording layer up in the direction of the arrow 140, seized by the pair of rolls and leaves the apparatus as shown by the arrow 150. The liquid developer sprayed onto the cleaning pad flows down along the surface of the cleaner and then of the development electrode rollers, reaching on the surface to be developed between the rollers 101-104 and the rollers 111-114. The toner on the development electrode roller deposits imagewise on the image forming surface leaving the residual toner pattern corresponding to the image on the roller. This residual pattern, which is harmful to succeeding development, is wiped off by the cleaning pad, hence the occurrence of the oflset image is prevented which would appear in the developed image by the transfer of the residual toner pattern on the roller.

The liquid developer may be sprayed directly into the surface of the development electrode rollers. The

developer, after utilized for development, flows downwardly entering into a reservoir not shown in the drawing, and then pumped by a pump also omitted from the drawing. The nozzle extends over the whole width of the development electrode roller, comprising a row of plurality of small aperture or a fine slit. If the cleaner is omitted, the development electrode roller has to be cleaned by an intense flow of the developer struck against the roller; however, such drastic flow of the developer affects adversely on the image quality causing streaks originating from a high density area.

The development electrode roller may be preferably made of metal, but it may be covered with rubber or plastic coating the surface of which is processed electrically conductive by suitable means. Metal rollers with surface insulating oxide layer, an insulating or semiconductive resinous coating, or with a coating comprising resinous binder and a pigment dispersed therein may also be used.

The surface of the development electrode roller may be processed smooth like a mirror or rough, depending on the surface structure of the electrophotographic sheet used. That is, for the sheet having a smooth surface, a rough surface electrode has to be used, while for the one with a rough surface, a smooth or rough electrode roller may be combined. When a smooth development electrode roller made of metal is used with and brought into contact with a smooth electrophotographic sheet, the electrostatic charge on the surface will leak away through the electrode roller, giving a developed image accompanied with mottle or spots as well as decreased saturation density. Thus, the resulting image is possessed of a very poor quality. Accordingly for a sheet with a smooth surface, a metal electrode roller with a rough surface is advantageously used. In case of a rough surface sheet, a smooth electrode roller is available. To sum up, it is essentially necessary for production of good quality image, that the roller and the surface to be developed be in contact with each other at finely divided spots uniformly distributed in the whole apparent contact area.

As is already known, microscopic observation of direct contact of a rigid material such as metal with another material discloses that the two materials only contact at a great number of separate small points. In the present invention the development electrode is contacted not only weakly, but in pressure contact engagement with the surface of the electrophotographic sheet as shown in the drawings so as to squeeze the liquid developer put between the electrode and the sheet.

The importance of the developer squeeze during development is explained as follows. A variety of conventional development methods are known in which the surface to be developed and an development elec-. trode roller are closely placed with a small spacing around several hundred microns. In the development unit shown in FIG. 6, which is an example of typical, conventional ones, the sheet is transported by the drum 11 with the photoconductive surface down, while the developer 2 is taken up by the rotating electrode 22, which is placed with a close spacing.

Our experiments and close observations disclosed that when a highly charged area of the sheet 1 passes near the electrode 22, all the toner included in the liquid film on the roller is instantaneously attracted on the charged area, thus leaving a clear toner-free liquid film. This clear layer, held on the surface of the sheet by its surface tension, remains stably standing the agitation by the rotating roller. The presence of such tonerfree film results in an image accompanied with an intense edge efiect as well as low saturation density.

Accordingly, to realize an ideal development of an electrostatic latent image it is essential to remove compulsorily the thin film held on the surface to refresh it with the toner containing liquid.

In the present invention, a development process is employed wherein the spacing between the electrode and the sheet is held zero in order to completely exclude the liquid film on the sheet during development and thus to provide a high saturation density image very rapidly.

FIG. 2 illustrates a schematic cross-sectional view of another developing device to carry out the present invention. In place of the cleaning pads 121-124 shown in FIG. 1, four squeeze rollers 201/205 are provided in this embodiment. The squeeze rollers are preferably soft, and resilient, suitable constructions include a metal roller having a coating of rubber or sponge. They may be either in pressure contact with the development electrode rollers by means of spring, or be set on the development electrode rollers simply by their own weights.

The squeeze roller functions similarly to the cleaning pads in FIG. 1, but is superior to the pad in the following respects; the pad is forced to the development electrode roller, thus undergoes gradually a permanent deformation in a prolonged use, emanating a lowering of cleaning efficiency. Such defect is not accompanied in case of the squeeze roller.

FIG. 3 illustrates another device in which an endless belt is employed in place of the carrier rollers in FIG. 1 and FIG. 2 to transport the electrophotographic material. The endless belt is driven along the path around the rollers 302 and 303.

The device shown in FIG. 4 employs a drum 401 in place of the endless belt. The electrophotographic sheet is fed into the device as shown by the arrow 410, and exhausted after processing in the direction shown by the arrow 420.

In the devices shown in FIGS. 3 and 4, the speed of the sheet is advantageously kept exactly constant during processing, while, on the other hand, the sheet will proceed in the circular velocity of each pair of rollers, resulting in a not-uniform development in case where each pair rotates at a slightly different speed from one another.

FIG. 5 illustrates another embodiment of the device to carry out the present invention. In this embodiment a plurality of endless belt electrode 501-507 are employed in place of the development electrode roller 101-104 of the device shown in FIG. 1. Each endless belt is driven around a pair of roller for example 511 and 521. The surface structure of these endless belts are processed to meet the requirements noted above.

In any of these embodiments, it is desirable to provide a cleaner or squeeze roller in order to clean each carrier roller or carrier belt and to avoid smudging of the back surface of the electrophotographic sheet.

As has been stated above, at least one of the development electrode and the electrophotographic sheet has to be possessed of a rough surface comprising fine, uniform irregularities.

As for the surface of electrophotographic sheet, the following results were obtained.

a. When the development electrode has a smooth surface; I

An electrophotographic sheet having a smooth surface proved to give a quality image only with very limited compositions of liquid developer and otherwise to suffer from destruction or fall-off of a once deposited toner image, resulting from back transfer of the image onto the development electrode. Further, when the surface of the electrode is conductive, irregular mottles appeared in the high density areas in the developed image perhaps because of the charge leakage through the intimately contacted points with the electrode.

On the other hand, a sheet having small irregularities throughout its surface hardly yield such defects in the image but provided a quality image stably with various compositions of liquid developer.

b. When the electrode has a rough surface;

The defects described above remarkably decreased with a sheet having a smooth surface and similarly good results were obtained with sheets having rough surfaces.

It is worth inserting here that the development electrode in the present invention is defined as a body which can effectively attract the electric lines of force originating in the electrostatic latent image formed on the electrophotographic sheet; accordingly, it may include, besides the typical metal electrode, conductive rollers or belts having on their surfaces a very thin layer of semi-conductive or insulator.

The following simple test can definitely judge whether a certain combination of an electrophotographic sheet and a development electrode such as shown in any of FIGS. 1 to 5 can give a desirable result.

The developing unit is set in its practical working conditions as for, for example, the roller pressure, the feed rate of the developer, the transport velocity of the sheet, etc. In the unit thus conditioned a sheet of electrophotographic paper is fed which has substantially no electrostatic charge thereon at all.

After the passage through the unit, the sheet is washed, if necessary, in a suitable rinse bath, dried and subjected to microscopic observation. When a black developer has been used, black spots, streaks or patterns can be clearly observed on the photoconductive surface. These areas correspond to the raised portion where the direct contact with the electrode occurred during passage. If such areas are uniformly distributed, most of them have a representative size not greater than 50 microns, and the areas sum up to not larger than percent of the total processed area, a desirable result is ensured since these areas are not perceived as separate points but as a uniform, sufficiently low background.

When a smooth, metal electrode is used for a smooth electrophotographic sheet the directly contacted area, though it is still finely-divided, amounts to a relatively large portion of the total area, and thus unfavorable affects the image quality, causing charge leakage and other disadvantages.

Preparation of an electrophotographic sheet having a rough surface adapted for the smooth electrode is elaborately carried out and will be described hereinafter. A preferable photoconductive layer 700 has raised portions or contact points 701 of 15 to 50 micron diameter, uniformly distributed, separated from each other by to 400 microns, and at the same time, the frequency of occurrence of coarse raised areas having a representative size larger than 100 microns does not exceed five times per 1 cm (more preferably two), and the sum of the raised portion which undergoes direct contact with the electrode amounts to not larger than 10 percent (3 percent in case where an extremely low background is required) in the total processed area.

Electrophotographic coatings comprising a finely-divided photoconductor such as ZnO, TiO ZnS, CdS, etc., and a resinous binder which meet the above requirements can be prepared by suitably regulating the dispersed state of the photoconductor in the binder. In case of ZnO binder coating, a coating having a surface roughness of about 30 to 50 microns can be obtained by blending rather coarse ZnO power synthesized by French process with a resin binder solution in a suitable blending condition with care of removing coarser agglomerates. Heat treatment of ZnO produced by oxidation of Zn vapor at near 600 C. provides a powder which has a poor affmity with many organic solvents and adapted to form a coating containing agglomerate of suitable size. Alternatively, one may make use of surface smoothness of support material to obtain an electrophotographic sheet meeting the above requirements. To ensure a high reproducibility of manufacture, paper having a well regulated smooth surface is favorable. Suitable types of paper include art paper, baryta coated paper for photography, machine coated paper, etc., which may subbed, if necessary, to form a solvent holdout barrier coating, and then coated an electrophotographic coating mixture containing 20 to 50 micron agglomerates.

These smooth supports can be used with the merit that the structure of the photoconductive coating does not depend on the coating thickness until the thickness decreases to 5 micron or less.

When a paper support comprises non-coated paper having a rough surface such as document paper, bond paper, or the like, electrophotographic coating mixture with or without agglomerates may be applied. Since with such support irregularities of paper arising from the entangled cellulosic fibers appear in the surface structure of the resulting photoconductive coating, and the raised portion due to raised fiber stretches laterally, which is observable when stained with toner, the thickness of the coating is preferably somewhat increased.

Also, one may resort to utilization of surface structure change during drying of the coating mixture on the support. Typical example is given by so-called orange peel pattern.

Coatings comprising organic photoconductor can be made suitable for the present invention by making their surfaces rough by incorporation of particulate ingredients. To maintain cleamess of the coating pigments the reflective indices of which are nearly equal to that of the photoconductor used. Suitable pigments include silica, magnesium oxide, aluminum oxide, calciuk silicate, etc. In case where there exists no need for film transparency, finely divided organic photoconductors such as are disclosed in the specification of Japan Pat. publication 43-27588 or a variety of inorganic pigments such as zinc oxide or titanium dioxide may be incorporated.

Two or more techniques described above can be combined, if possible. Experiments accomplished the lower limit for direct contact portion relative to the total area of about 0.1 percent.

As is already known, the background which is sometimes associated with electrophoretic development can be noticeably decreased by wetting the surface to be developed with a toner-free insulating liquid prior to development on account of decrease of toner deposition onto the surface by the forces other than electrostatic. Such pre-treatment or pre-bathing can, of course, be utilized in the present process and quite effective especially when an electrophotographic sheet with a rough surface which is ready to attract and entrap toner particles. Further rinse operation of the sheet after development to remove the excessive developer liquid adhering on the surface is also efiective to decrease background.

There may be used for the present process the technique for improving the mechanical durability of the developed image so as to stand squeezingoperation, comprising utilizing a liquid developer in which a resinous material is dissolved in the carrier liquid and rinsing the developed sheet with a rinse liquid in which the resinous material is insoluble.

Now will be given several examples in which part will be given by weight if otherwise mentioned.

EXAMPLE I.

A mixture of 700 parts of photoconductive zinc oxide having a mean particle size of 0.62 micron and 300 parts of another grade of photoconductive zinc oxide having a mean particle size of 1.98 micron, were added 240 parts of styrenated alkyd resin varnish purchased from Japan Reichhold Chemical Co. under the trade name Styresol 4400, 110 parts of polyisocyanate compound varnish purchased from Bayer in West Germany under the trade name Desmodur L and a suitable amount of n-butyl acetate, and blended in a homogenizing mixer for about 30 minutes. The resulting dispersion was filtered through a 200 mesh shieve screen to eliminate coarse particles and particulate contamination.

Paper for support was prepared in the following procedure; on the back side of baryta paper for photographic use was coated an electrically conductive paint comprising carbon black and an aqueous emulsion of polyvinylacetate to give a dried coating weight of 3 gs per square meter; then the baryta layer was impregnated with an aqueous solution of potassium polyvinyl benzene sulfonate.

The filtered coating mixture prepared above was applied on the treated baryta layer to give a dried thickness of about 6 to 7 microns.

The curing and drying of the coating was done for 16 hours at 50 C. The photoconductive coating thus prepared looked dull, and the microscopic observation disclosed the presence of uniformly distributed agglomerates with a diameter of 30 to 50 microns throughout the surface with the frequency of coarse granules greater than 100 microns less than once per 1 square cm.

For the purpose of comparison, on the same support was provided a glossy photoconductive coating comprising 1000 parts of the finer powder of zinc oxide (0.62 u. mean particle size) and the same resin composition. Observation under microscope with 120 magnification could not discriminate any irregularities due to agglomerates on the surface of this sample. These two kinds of electrophotographic material will be referred to as matt and glossy ones, respectively. A sheet of each kind of paper was charged to 120 volts at completely dark-adapted state by means of corona, contact exposed to a positive transparency and subject to electrophoretic development manually in a stainless tray with the recording surface down so as to face to the bottom of the tray which worked as development electrode. An image of similar quality was obtained on each sheet. This means that no charge leakage during contact exposure occurred. Then another sheet of each paper was, after charged and image exposed in the same manner, subjected to development with the development unit shown in FIG. '1. Prior to development pretreatment of the photoconductive surface was carried out by wetting with pure kerosene. The

development electrodes 101 to 104 consisted of stainless steel roller having a mirror-polished surface, while the backup rollers 111 to 114 were metal rollers covered with a hard rubber. After the passage through the unit the sheet was rinsed with an isoparaffinic solvent and dried.

The developer used was made by dispersing carbon black in kerosene. There was formed a satisfactory quality image on the matt paper, while on the glossy paper a number of low density mottles appeared in high density areas.

The test described in the above explanation gave the sum of the directly contacted area of the matt paper with the electrode of 1.5 percent of the total area. And the individual points of the contacted area had an area ranging from 20 to 40 microns.

On the other hand, the glossy paper gave a dense background throughout the total area showing a too intimate contact with the electrode.

EXAMPLE II.

Photoconductivezinc oxide having an average particle diameter of 1.3 micron synthesized by French process and further heat treated at 600 C. for a short period was used as photoconductor in this example. 1000 parts of this zinc oxide, 400 parts of epoxyester of linseed oil fatty acid purchased under the trade name Epikosol 803-MS from Japan Coating Co., and a suitable amount of xylol were charged in a porcelain ball mill and mixed. The resulting dispersion was filtered and added 2 parts of cobalt naphthenate. The support material was prepared in the following manner.

A high-grade printing paper PHO of Fuji Photo Film Co., having a thickness of microns was treated at one surface with an aqueous dispersion of colloidal alumina, and at the other side with an aqueous solution containing potassium salt of polyvinyl benzene sulfonic acid and glycerol in an equal quantity to form a subbing, conductive barrier layer.

The coating mixture prepared above was applied on the sub layer to give a dried thickness of about 8 microns. The surface of the photoconductive coating had small raised points with about 50 micron diameter throughout the surface. A sheet of this paper was charged 80 volts in a completely dark-adapted state and processed in a similar manner as described in Example I, with an acceptable result.

The testing procedure disclosed a direct contact area of 2 percent, with the individual points of about 25 ITllCl'OnS.

EXAMPLE III.

650 parts of fine grain zinc oxide (average diameter 0.4 micron), 350 parts of coarse grain zinc oxide (average diameter 1.3 micron), 240 parts of styrenated alkyd resin varnish used in Example I, 110 parts of polyisocyanate varnish used in Example I were charged in a homogenizer; further there were added as spectral sensitizer 0.2 part of Food blue 1 (Color Index 42090), 0.4 part of Eosine (CI. 45380), and Fluorescein (CI. 45350) dissolved in 40 parts of methanol.

The resulting mixture was mixed for about 30 minutes with a suitable amount of n-butyl acetate and xylol. The homogeneous dispersion was coated on the same paper as described in Example I.

The electrophotographic sheet produced had a nonglossy surface and proved to give a quality image with a developing unit shown in FIG. 2 or FIG. 4 having smooth electrode rollers.

The direct contact area occupied only 0.5 percent of the total surface, the individual points being to 30 microns in size.

EXAMPLE IV.

A developing unit having a structure shown in FIG. 3 was employed. The development electrodes 101 to 104 were made of stainless rollers having a sandblasted rough surface with a roughness corresponding to JIS 35-5.

An electrophotographic sheet was prepared in a similar manner described in Example IH whereby 1000 parts of Sazex 2000, which is a grade of photoconductive zinc oxide famous for its high dispersibility in organic resin solutions and having an average particle size of 0.6 micron, were used. The resulting photoconductive paper had a very smooth, glossy surface in which no irregularities were observed by the microscopic observation with a magnification of 120 except raised areas of about 5 microns scattered with a spacing of about 100 microns.

A sheet of this paper was charged negatively (to an initial potential of about 100 volts), exposed by means of a reflection exposure system to a continuous tone image formed on a sheet of B/W photographic paper, and developed electrophoretically. A developed image resulted which was free of mottles in the high density areas.

Microscopic observation disclosed the presence of a small black spots less than is micron diameter which corresponded to the raised portions of the stainless development electrode rollers. The total area of these spots was about 4 percent in the whole processed area.

EXAMPLE v.

The paper described in Example III was treated in the device of FIG. 1. The development electrodes were composed of surface treated stainless steel rollers having an oxide film made by the treatment with the solution comprising sulfuric acid and potassium bichromate. The surface roughness of these rollers were expressed as I IS l40-S.

What is claimed is:

1. An electrophotographic process comprising:

1. providing a photoconductive insulating layer;

2. providing an electrically conductive development electrode;

3. fonning an electrostatic latent image on said photoconductive insulating layer;

moving said photoconductive insulating layer by said development electrode in such a manner that the surfaces of said photoconductive insulating layer and said development electrode (a) undergo no relative movement and (b) directly contact one another at no more than 10 percent of the whole area of the processed surface to be developed of said photoconductive insulating layer where the majority of the points of contact are substantially uniformly distributed and have a diameter not greater than 50 microns; and i 5. developing said electrostatic latent image by introducing developing liquid containing charged toner particles between said development electrode and said photoconductive insulating layer so that said developing liquid contacts said photoconductive insulating layer at both the image and nonimage areas thereof whereby said toner particles are attracted only to the electrostatic latent image areas to thereby effect the reproduction of line and continuous tone images whereby the edge effect in the reproduced image is substantially minimized because of the said direct contact between the photoconductive insulating layer and the development electrode.

2. A process as in claim 1 where the majority of said contact points are separated from each other by to 400 microns.

3. A process as in claim 1 where the density of said points of contact which have a diameter greater than 100 microns does not exceed 5 per square centimeter.

4. A process as in claim 3 where said density does not exceed 2 per square centimeter.

5. A process as in claim 1 where said contact points directly contact one another at no more than 3 percent of the whole area of the processed surface to be developed.

6. A process as in claim 1 where the surface of said photoconductive insulating layer is smooth and the surface of said development electrode is rough.

7. A process as in claim 1 where both the surfaces of said photoconductive insulating layer and said development electrode are rough.

8. A process as in claim 1 where the surface of said photoconductive insulating layer is rough and the surface of said development electrode is smooth.

9. A process as in claim 1 where said development electrode comprises at least one roller.

10. A process as in claim 1 where said development electrode comprises at least one endless belt.

=0 III 

2. A process as in claim 1 where the majority of said contact points are separated from each other by 100 to 400 microns.
 2. providing an electrically conductive development electrode;
 3. fOrming an electrostatic latent image on said photoconductive insulating layer;
 3. A process as in claim 1 where the density of said points of contact which have a diameter greater than 100 microns does not exceed 5 per square centimeter.
 4. A process as in claim 3 where said density does not exceed 2 per square centimeter.
 4. moving said photoconductive insulating layer by said development electrode in such a manner that the surfaces of said photoconductive insulating layer and said development electrode (a) undergo no relative movement and (b) directly contact one another at no more than 10 percent of the whole area of the processed surface to be developed of said photoconductive insulating layer where the majority of the points of contact are substantially uniformly distributed and have a diameter not greater than 50 microns; and
 5. developing said electrostatic latent image by introducing developing liquid containing charged toner particles between said development electrode and said photoconductive insulating layer so that said developing liquid contacts said photoconductive insulating layer at both the image and non-image areas thereof whereby said toner particles are attracted only to the electrostatic latent image areas to thereby effect the reproduction of line and continuous tone images whereby the edge effect in the reproduced image is substantially minimized because of the said direct contact between the photoconductive insulating layer and the development electrode.
 5. A process as in claim 1 where said contact points directly contact one another at no more than 3 percent of the whole area of the processed surface to be developed.
 6. A process as in claim 1 where the surface of said photoconductive insulating layer is smooth and the surface of said development electrode is rough.
 7. A process as in claim 1 where both the surfaces of said photoconductive insulating layer and said development electrode are rough.
 8. A process as in claim 1 where the surface of said photoconductive insulating layer is rough and the surface of said development electrode is smooth.
 9. A process as in claim 1 where said development electrode comprises at least one roller.
 10. A process as in claim 1 where said development electrode comprises at least one endless belt. 